The present invention relates to a thermosensitive semiconductor device which has a high thermo-sensitivity and can be manufactured with high yield.
A transducer for converting a temperature value into an electric signal, that is, a temperature sensor is used in various fields today, and the demand for such a sensor continues to increase. For example, in our daily lives, such a sensor is used in an air-conditioner (heating fan, cooler), refrigerator, electric heater, electric blanket, electric range, electric oven, and the like.
For contact type thermo-sensors, a bi-metal, a thermistor, a p-n junction semiconductor, a resistor for temperature measuring, a thermocouple, a liquid thermosensor, a p-n junction transistor or the like have been used.
The P-N junction semiconductor is the most suitable of all devices for use as a temperature sensor which can be effectively manufactured by a mass-production system and by the use of integrated circuit techniques. One example of the structure of such a thermosensitive IC is illustrated in FIG. 1. In this case, an IC using an n-type Si substrate will be described.
A constant current circuit 2 for providing a constant current (e.g. about 0.1 .mu.A) is connected to a junction transistor 1 to cause a constant current to flow through the transistor 1, to thereby form a thermosensor and the voltage between the base-emitter of the junction transistor is produced as a temperature detected voltage V.sub.out. In FIG. 1, 3 is a power source circuit including a voltage regulating circuit for providing a constant voltage regardless of changes in the source voltage. In order to reduce the power consumption in the constant current source 2 and the voltage regulating circuit, these circuits are formed in the form of an integrated circuit by the use of the complementary type insulation gate field effect transistor (referred to as C-MOS hereinafter), and an n-p-n planar type junction transistor, which has a structure as shown in FIG. 2, can be formed in the same manufacturing process as that for the C-MOS and used as the junction transistor 1.
FIG. 2a is a sectional view illustrating a structure of the most basic NPN planar Si junction transistor and FIG. 2b is a plan view thereof as viewed along section line A--A. That is, in this transistor, an N.sup.- Si substrate 4 acts as the collector, a P.sup.- Si region 5 acts as the base and an N.sup.+ diffusion layer 8 acts as the emitter. The equivalent circuit of the transistor is illustrated in FIG. 2c. As shown in FIG. 2a, 7 is a P.sup.+ channel-cut region, 6 is an N.sup.+ diffusion region, 9 is an oxide insulation film, 10 is a metal electrode, 11 is a base-collector terminal and 12 is an emitter terminal.
When the collector 4 and the base 5 of the junction transistor are at the same potential, the characteristic of the emitter current I will be the same as the current-voltage characteristic of a diode and it can be expressed by the following equation. ##EQU1## where, n.sub.i : intrinsic carrier density
n: emitter injection efficiency PA1 R: Boltzmann's constant, PA1 e: unit charge, PA1 V: voltage between base and emitter PA1 A: constant which depends upon the configuration of the junction transistor and the diffusion length of the minority carriers.
The current-voltage characteristics between the base 5 and the emitter 8 correspond to the characteristics shown in FIG. 3, and each characteristic curve depends upon the changes in temperature. T.sub.1, T.sub.2 and T.sub.3 represent different temperature values and have the relationship of T.sub.1 &gt;T.sub.2 &gt;T.sub.3. As described above, by the use of the CMOS process, a thermosensing element can be easily fabricated on a chip at the same time the constant current circuit and voltage regulating circuit are fabricated, and it is widely used as a one-chip thermosensor for various controls. In this device, the temperature coefficient of 3 mV/.degree.C. can be obtained for a P.sup.- N.sup.+ junction area of 100.times.100 .mu.m.sup.2, P.sup.- well concentration (base diffusion concentration) of 5.times.10.sup.15 atoms.cm.sup.-3 and a constant current of 0.1 .mu.A in the forward direction. There is the disadvantage that the thermosensitivity of the thermosensor using such a junction transistor is lower than the thermosensitivity (15 mV/.degree.C.) of the thermosensor using a thermistor.